The spark plug is attached to an internal combustion engine for example, and is used to ignite a mixture within a combustion chamber. Generally, the spark plug includes an insulating body that has an axial hole, a center electrode that passes through a leading end side of the axial hole, a metal shell that is provided at the outer periphery of the insulating body and a ground electrode that is provided at the leading end section of the metal shell and forms a spark discharge distance between the center electrode and ground electrode. Also, generally, the metal shell includes a tool engaging section in which a tool or the like is engaged when the metal shell is attached to the internal combustion engine or the like and a seat section that is attached with respect to the engine head of the internal combustion engine directly or indirectly through a gasket or the like.
However, the metal shell and the insulating body are fixed by clamping and then assembled. More specifically, in a case where the insulating body is inserted into the cylindrical metal shell, a load is applied with respect to a rear end side opening of the metal shell along the axial direction by a circular mold. Thus, the rear end side opening of the metal shell is bent to the inside in the diametrical direction and becomes a clamping section that is engaged to a large diameter section that bulges to the outside in the diametrical direction in the insulating body, and the metal shell and the insulating body are assembled.
Also, as a method of fixing in clamping, so-called clamping by heat is known (for example, see Patent document 1). In other words, while the load is applied by the mold, the metal shell is heated by electrical conduction through the mold and a relatively slender middle section that is positioned between the tool engaging section of the metal shell and the seat section is heated. Thus, when the deformation resistance of the middle section is small, the middle section is by the load and buckles inward. After that, the middle section that is in a heat expansion state is cooled and contracted so that the clamping section of the metal shell becomes in an strongly engaged state with respect to the large diameter section of the insulating body; and the insulating body and the metal shell are strongly assembled.